In British Patent Specification No. 1,481,683 (which is assigned to the assignee of the present application) an ignition system for an internal combustion engine is described in which the timing of spark ignition is controlled by an electrical circuit that includes a memory programmed with information about the desired spark timing as a function of engine speed and engine load. The information in the memory is addressed with signals indicative of engine speed and load in order to derive from the memory an output word indicative of the desired spark timing. This arrangement permits a more accurate control of spark timing than has hitherto been possible with mechanical spark timing control arrangements, and consequently aids in allowing factors such as fuel economy, pollutant emission and performance to be optimised for the engine. The optimum spark timing is a complex non-linear function of speed and the load and in order to store this function in the memory it has previously been proposed that for each value of engine speed there should be stored a group of timing values for different values of engine load. It will therefore be appreciated that a large capacity memory with complicated addressing circuitry is required so as to store an adequate number of timing values over the operating ranges of engine speed and load, in order to provide adequate accuracy and resolution in the timing values produced by addressing the memory, to warrant use of the circuit rather than the conventional mechanical arrangement.